The present invention relates to the field of optical fiber transmission, and more specifically it relates to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
The refractive index profile of an optical fiber is generally described in terms of the appearance of a graph plotting the refracting index to the fiber as a function of radius. The distance r from the center of the fiber is conventionally plotted along the abscissa, and the difference in refractive index relative to that of the cladding of the fiber is plotted up the ordinate. The term xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d, and xe2x80x9ctrianglexe2x80x9d are therefore used with respect to index profiles whose graphs are respectively step-shaped, trapezium-shaped, and triangular. These curves are generally representative of the ideal or theoretical profile for the fiber, and fiber manufacturing constraints can yield a profile that departs perceptibly therefrom.
In new high bit rate transmission networks that are in wavelength division multiplex (WDM), it is advantageous to manage chromatic dispersion, in particular for bit rates faster than or equal to 10 gibabits per second (Gbit/s); the idea that for all wavelength values of the multiplex chromatic compensation should accumulate to substantially zero over the link as a whole, so as to limit the extent to which pulses widen. Over an entire transmission system, it is acceptable for the cumulative value of dispersion to be a few hundreds of picoseconds per nanometer (ps/nm). It is also beneficial to avoid zero values for chromatic dispersion in the vicinity of the wavelengths actually used in the system since that makes them more subject to non-linear effects. Finally, it is also beneficial to limit the chromatic dispersion slope over the wavelength range of the multiplex so as to avoid or limit distortion between the channels of the multiplex. This problem of compensating chromatic dispersion and chromatic dispersion slope is particularly severe with very high bit rate transmission systems, typically for WDM transmission systems having per channel rates of 40 Gbit/s and above. The problem becomes more severe with bandwidth increasing up to values greater than or equal to 30 nanometers (nm) or even to 35 nm.
Conventionally, the line fibers used in optical fiber transmission systems are step-index fibers; these fibers are commonly referred to as single-mode fibers (SMFs). Thus, the Applicant markets a step index monomode fiber under the reference ASMF 200 which presents a chromatic dispersion cancellation wavelength xo lying in the range 1300 nm to 1320 nm, chromatic dispersion of 3.5 picoseconds per nanometer kilometer (ps/(nm.km)) in a range from 1285 nm to 1330 nm, and of 18 ps/(nm.km) at 1550 nm. Its chromatic dispersion slope at 1550 nm is about 0.06 picoseconds per square nanometer-kilometer ps/(nm2.km). In known transmission systems, that fiber is used for transmitting signals with wavelengths close to 1550 nm (band C).
In that band, in order to compensate chromatic dispersion and chromatic dispersion slope in SMFs or in non-zero dispersion-shifted fibers (NZ-DSFs) used as line fibers, it is known to use short lengths of dispersion-compensating fiber (DCF). DCF has chromatic dispersion and chromatic dispersion slope of sign opposite to that of the chromatic dispersion and chromatic dispersion slope in the line fiber. An example for an SMF line fiber is given by L. Gruner-Nielsen et al., in xe2x80x9cLarge volume manufacturing of dispersion-compensating fibersxe2x80x9d, OFC""98 Technical Digest TuD5.
EP-A-0 935 146 proposes dispersion-compensating fibers adapted to compensating chromatic dispersion and chromatic dispersion slope in SMFs, over a range of wavelengths around 1550 nm. Around that wavelength, such fibers present a ratio of chromatic dispersion over chromatic dispersion slope that is close to the ratio of chromatic dispersion over chromatic dispersion slope for the line fiber. That document proposes various fiber profiles; its FIG. 3 shows a fiber having a rectangle index profile with a buried trench and a ring.
Compared with that document, the present invention seeks to solve the novel problem of compensating chromatic dispersion in SMFs in band S. The term xe2x80x9cband Sxe2x80x9d is used herein to mean the band which extends from 1450 nm to 1500 nm or from 1460 nm to 1490 nm, or around 1475 nm. Increasing interest is being shown in this band given the increase in the number of channels in terrestrial WDM systems. The invention proposes a solution adapted to high bit rate transmission over large bandwidths in existing transmission systems.
More precisely, the invention proposes an optical fiber that is monomode at 1475 nm and, at said wavelength, presents chromatic dispersion of less than xe2x88x9240 ps/(nm.km), a ratio of chromatic dispersion over chromatic dispersion slope less than 250 nm, and an effective section area greater than or equal to 14 xcexcm2.
Advantageously, it presents an effective area greater than 13 xcexcm2 at 1450 nm. It can also present, at 1475 nm, chromatic dispersion greater than or equal to xe2x88x92150 ps/(nm.km).
In an embodiment, the fiber presents, at 1475 nm, chromatic dispersion less than or equal to xe2x88x9260 ps/(nm.km). The fiber can also present, at a wavelength of 1475 nm, a ratio of chromatic dispersion over chromatic dispersion slope lying in the range 170 nm to 230 nm.
In another embodiment and at a wavelength of 1500 nm, the fiber presents bending losses less than 10xe2x88x923 dB for a coil of 100 turns of fiber on a radius of 30 mm. It can also present bending losses less than 100 dB/m at a wavelength of 1500 nm for a loop of fiber having a radius of 10 mm.
Preferably, the fiber presents, for a wavelength of 1475 nm, attenuation less than 1.2 dB/km. In yet another embodiment, the fiber presents, for a wavelength of 1475 nm, a mode diameter greater than 4 xcexcm. It can also present, for a wavelength of 1475 nm, sensitivity to microbending less than 1, and preferably less than or equal to 0.5.
It is also advantageous for the fiber to present a theoretical cutoff wavelength longer than 1100 nm, and shorter than 1800 nm, preferably shorter than 1700 nm, or even shorter than 1600 nm.
Concerning its profile, the fiber can present a rectangle index profile with a depressed trench and a ring, or a trapezium index profile with a depressed trench and a ring.
In an embodiment, the difference between the index of the rectangle or of the ring and the index of the cladding lies in the range 16xc3x9710xe2x88x923 to 25xc3x9710xe2x88x923, and the radius of the portion of the fiber presenting an index greater than or equal to that of the cladding lies in the range 1.3 xcexcm to 2.3 xcexcm.
In another embodiment, the difference between the index of the depressed trench and the index of the cladding lies in the range xe2x88x929xc3x9710xe2x88x923 to xe2x88x925xc3x9710xe2x88x923, and the outside radius of the trench lies in the range 3.7 xcexcm to 6 xcexcm.
In yet another embodiment, the difference between the index of the ring and the index of the cladding lies in the range 3xc3x9710xe2x88x923 and 11xc3x9710xe2x88x923, and the outside radius of the ring lies in the range 6.6 xcexcm to 8.3 xcexcm.
Preferably, twice the integral of the product of the radius multiplied by the index between radius zero and the outside radius of the central portion of the fiber presenting an index greater than that of the cladding lies in the range 30xc3x9710xe2x88x923 xcexcm2 to 60xc3x9710xe2x88x923 xcexcm2.
It is also possible to provide for the product of the square of the outside radius of the depressed trench multiplied by the index of the depressed trench to lie in the range xe2x88x92300xc3x9710xe2x88x923 xcexcm2 to xe2x88x92110xc3x9710xe2x88x923 xcexcm2.
It is also possible for the product of the thickness of the ring multiplied by the index of the ring to lie in the range 7xc3x9710xe2x88x923 xcexcm and 14.5xc3x9710xe2x88x923 xcexcm.
The invention also proposes a transmission system in which the line fiber comprises a step index monomode fiber which is dispersion-compensated in band S. It is advantageous for the cumulative chromatic dispersion for each channel in the range 1460 nm to 1490 nm to have an absolute value of less than 100 picoseconds per nanometer (ps/nm) on average for transmission over a distance of over 100 kilometers (km).
The line fiber can be constituted by a step index monomode fiber or by a step index monomode fiber and a dispersion-compensating fiber. The above-mentioned fiber is particularly advantageous for use in such a transmission system as the dispersion-compensating fiber.
Finally, the present invention proposes a dispersion-compensating module comprising an amplifier and a segment of such a fiber.